1. Field of the Invention
The present invention relates to a vehicle height adjustment system for adjusting the vehicle height of a vehicle.
2. Description of the Related Art
Large vehicles such as a heavy-duty truck should desirably keep the height of a vehicle body frame (vehicle height) at a predetermined height from an axle (reference vehicle height) irrespective of the weight of loaded cargos, in order to secure a given suspension stroke to provide the ride comfort and to realize efficient reduction in vibrations of a cargo bed. Against this backdrop, these vehicles generally have air suspensions using an air spring as a chassis spring and adjust the vehicle height by controlling supply and discharge of air to/from the air spring.
Operability of the vehicle is further improved with quicker adjustment of the vehicle height to the reference vehicle height. However, such quick adjustment of the vehicle height during vehicle running is likely to disadvantageously affect running stability of the vehicle. To overcome this problem, conventional vehicle height adjustment systems have been designed to improve the running stability of the vehicle by, in a running state of the vehicle, adjusting the vehicle height to the reference vehicle height at a restoring speed slower than that in a stopped state of the vehicle (for example, see the Patent Document 1). That is, such vehicle height adjustment systems have a stop mode for high-speed adjustment of the vehicle height and a running mode for low-speed adjustment of the vehicle height, either of which is activated depending on the vehicle speed.
(Patent Document 1) Japanese Patent Laid-Open No. 2001-47837
However, the vehicle height adjustment system having such a structure as the above has its own problem. When the vehicle is started with a vehicle height inappropriate for running, the vehicle goes into the running mode for low-speed adjustment of the vehicle height, and thus the vehicle height inappropriate for running is kept for a long period of time, which probably hinders traveling of the vehicle.